1. Field of the Invention
This invention relates to image editing, and more particularly to correcting shading noise.
2. Description of the Prior Art
In general, a scanning process of a scanner includes: moving a light source and a scanning module along a scanning direction by means of a stepper motor. Then, the light source illuminates the material of demand and is reflected into the scanning module. An image is captured by a light sensor such as CCD (Charge-Coupled Device) and is then translated into digital data to be saved.
The digital data of the image is different from the captured data with the light sensor, due to the light provided by the light source not being an ideal linear light and the brightness varying along a direction of the arrangement of the light sensor. The brightness is approximately brighter in the center of the light than in the edge thereof, as shown in FIG. 1A. Therefore, the captured image is brighter in the center of the image than in the edge thereof due to a disproportionate brightness of the light source. In general, the brightness distribution of the light is pre-scanned and the captured data is efficiently corrected through use of the pre-scanned data, as shown in FIG. 1A.
Furthermore, a light sensor is a plurality of CCDs arranged in a line and the data detected by different CCDs is different due to each COD having a different light sensitivity. Moreover, external factors may vary with time, for example: the variant of light resulting from a power supply varying with time, the variant in the light sensitivity of the CCD with time, etc. These factors are generally called “shading distortion”. Therefore, the data of the image is corrected for the elimination of the disproportionate distribution of the light, but the corrected data still has a problem of being uneven, resulting from the shading noise, as shown in FIG. 1B.
The corrective method of the shading distortion is that the scanner pre-scans a reference white (a white corrective board) or a reference black (covering with the light sensor before capturing image). The data of the reference white or the reference black is the shading distortion corrective curve. Hence, a captured image can be corrected with the corrective curve to obtain a more corrective image data.
Nevertheless, the shading distortion corrective curve has local maximums or local minimums resulting from the noise of the electrical devices, bad CCDs, or inexact deductive methods. The local maximums or minimums resulting from random noise do not appear every time. The inexact deductive method causes the data to be improperly corrected wrong. These results are generally called “shading noise”. If the image data is corrected by a corrective curve that comprises shading noise, the resultant corrected image has some shading noise lines. The shading noise lines in the corrected image are caused by the shading corrective curve. While the shading corrective curve may be one dimensional the shading noise that appears in the resultant corrected image appears as lines in a two-dimensional image in positions correlating to the shading distortions in the corrective curve.
Hence, the shading corrective curve in the conventional arts can correct the problem of shading distortion, but they easily generate problems with shading noise and reduce the quality of the image.